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Thread: What "Planet clears its neighborhood" really means.

  1. #121
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    Quote Originally Posted by swampyankee View Post
    Just out of curiosity, which planets?
    Circumstellar disc resolved: since at least 1984 (Vega). Over 900 examples by 2001 already.
    Structure found in disc, suspected to be cleared into disc by planet - also several, including Vega.
    Associated planet spotted: certainly Fomalhaut b. Probably others.
    Last edited by chornedsnorkack; 2017-Mar-02 at 07:00 PM.

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    Taxonomy used to be all about physical appearance: a crow's physical appearance is quite distinct from that of a stork. Lately, taxonomic classification has been modified by DNA, and some species have been moved/eliminated/added. Physical appearance is determined largely by genetics, so it's an intrinsic property: no matter what the environment a crow won't look like a pileated woodpecker. Location does not: biologists don't deliberately place an organism from Maine into a different category than an identical organism from Nova Scotia: location is not an intrinsic property of an organism.

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    Quote Originally Posted by chornedsnorkack View Post
    Circumstellar disc resolved: since at least 1984 (Vega). Over 900 examples by 2001 already.
    Structure found in disc, suspected to be cleared into disc by planet - also several, including Vega.
    Associated planet spotted: certainly Fomalhaut b. Probably others.
    Protoplanets are something much different than anything intended in the definition. Vega seems to be an odd case of a possible collision that produced a debris disk.
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    Quote Originally Posted by swampyankee View Post
    Taxonomy used to be all about physical appearance: a crow's physical appearance is quite distinct from that of a stork. Lately, taxonomic classification has been modified by DNA, and some species have been moved/eliminated/added. Physical appearance is determined largely by genetics, so it's an intrinsic property: no matter what the environment a crow won't look like a pileated woodpecker.
    That's logical. Taxonomy seems to have... evolved a bit, and tweaking is where science shines above many other fields. It applies to planet labeling, too, apparently. I guess that should be "re-labeling" since Ceres and a few others were demoted when asteroids became known. So maybe the biology analogy helps. Further, you mention that it has been lately that DNA is the driver for naming. But there must be a number of species that remain unchanged that do refer to more outward appearances or activity. A couple of minutes on Google has made me the worst kind of expert, but perhaps a few examples will help me understand it a little better...

    Take megadyptes antipodes "yellow-eyed penguins". Megadyptes, apparently, is a word for "large diver". I'm not sure about "antipodes", I want to say it refers to poles, but I want to keep my errors to a minimum. Ursus maritmus (polar bears) refers to its marine life activity, the most for any bear. Neither refer to specific locations, but it seems to me there may be many formal names that are heavy on physical activity and, perhaps, appearance. This, however, may just be some historical carry-over, but history seems to be a very powerful factor in defining what a planet is and is not.

    Location does not: biologists don't deliberately place an organism from Maine into a different category than an identical organism from Nova Scotia: location is not an intrinsic property of an organism.
    Yes, I recognized that was your original point. I was using the analogy loosely because there seems to be a lot of bending action needed to get the planet definition nailed effectively.
    Last edited by George; 2017-Mar-02 at 07:25 PM.
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    Quote Originally Posted by Ara Pacis View Post
    Arbitrariness shouldn't bother the neighborhood clearer crowd, since their choice is also arbitrary. Pick the amount of clearing you want in the time frame you want, and do the math and there's your result, but it's still arbitrary.
    So would a few percent variance between 1 billion and 4 billion years be arbitrary?

    I suspect the mass-roundness crowd could come up with something usable. But just like neighborhood clearing, initial observation might not be enough (unless you're in a starship that can drop out of warp drive right next to it). But if you want a comprehensive definition, it'll probably include a balance of forces, so that it's as round as gravity, rotation and compositional fluidity and internal dynamics allow. If the object has tectonic or volcanic dynamics that create a geologic excursion well above or below datum, then that would be allowed as part of the dynamics. If a large object with a datum in the clouds has an unbalanced atmosphere because of proximity to the sun or another object then that object would be allowed. If an object rotates in a manner that creates oblateness or loblateness, then that would be allowed, considering that fluidity of the material.

    A lot of this wouldn't be discernible from a distance immediately (just like clearing criteria), so rules of thumb will develop that use proxies. As others have said, clearing of a neighborhood can be used to discern mass, from which roundness can be estimated. So can induced wobble in a star. Spectroscopy of the object, when possible, would reveal surface or atmospheric chemistry, from which internal processes might be predicted that leads to an estimate of it's mass and roundness.
    In a thousand years of technological advancements, this should be considered. Hopefully, before then, we can ask a more advanced civilization how they define planets.

    But if we're gonna use arbitrary criteria, then we may as well make them human-relatable. How about a minimum g that will allow a human to walk without kicking themselves into orbit or escape velocity.
    How big a kick? A Pogo stick should be used, with an accurate one kept in Paris.
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    Quote Originally Posted by George View Post
    Protoplanets are something much different than anything intended in the definition. Vega seems to be an odd case of a possible collision that produced a debris disk.
    Yet Sun has zodiacal light. And Ceres is classified as a dwarf planet rather than a protoplanet.

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    Quote Originally Posted by chornedsnorkack View Post
    Yet Sun has zodiacal light. And Ceres is classified as a dwarf planet rather than a protoplanet.
    I don't see how this would "throw debris" at the current definition. A little cleared ring in a debris reflection may reveal a planet or it may not. For protoplanetary disks, I doubt it would even be considered as far as any formal planetary definition would apply. A billion + year requirement is recommended and needed to allow the clearing to have been essentially completed, though the time constraint ties back to the mass, so waiting that entire time would not be necessary since the physics reveals what we need.
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    Quote Originally Posted by George View Post
    For protoplanetary disks, I doubt it would even be considered as far as any formal planetary definition would apply. A billion + year requirement is recommended and needed to allow the clearing to have been essentially completed, though the time constraint ties back to the mass, so waiting that entire time would not be necessary since the physics reveals what we need.
    Many planets are younger than that. Such as, e. g., Pollux b.

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    Quote Originally Posted by George View Post
    So would a few percent variance between 1 billion and 4 billion years be arbitrary?
    No, the percentage of extraneous masses removed to make the orbit "clear" is arbitrary: 90, 95, 99, 99.9, 99.9999 percent. The IAU didn't place a value in their definition.

    In a thousand years of technological advancements, this should be considered. Hopefully, before then, we can ask a more advanced civilization how they define planets.
    Or maybe they don't consider the definition useful, and don't have one.

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    Quote Originally Posted by swampyankee View Post
    No, the percentage of extraneous masses removed to make the orbit "clear" is arbitrary: 90, 95, 99, 99.9, 99.9999 percent.
    I'm not convinced this is that problematic, but it raises the right question...can clearing be quantified to an acceptable degree? Perhaps someone will show variances with different clearing times and masses. It may be a few days before I get free time to do so.
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  11. #131
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    As can be heard in the Session II video, the third one in the presentation linked above, the original language of the proposed resolution was about gravitational dynamic dominance. The group responsible for the drafting of the resolutions chose to change that to "clearing" in an attempt to make it more easily understood by the general public, but it appears that the attempt was not successful and has complicated things instead of simplifying them. Some of the delegates appeared to be confused, though it is sometimes hard to tell whether a particular delegate was really confused or was being some sort of a devil's advocate.

    It's the nature of the beast that this criterion requires explanation in detail that is mathematically beyond the working knowledge that most of us have in orbital mechanics. My OP in this thread is aimed at the general public, and I stand by my opinion that presenting it that way is better than simplistically saying "has cleared its orbital neighborhood". It takes more time and effort but gives the public an honest statement. Those who don't understand the dynamic basis of it but wish to learn more can enroll in a suitable astronomy major curriculum. Trying to learn it by browsing online can be dicey because of the risk of encountering garbage.

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    Quote Originally Posted by Hornblower View Post
    As can be heard in the Session II video, the third one in the presentation linked above, the original language of the proposed resolution was about gravitational dynamic dominance. The group responsible for the drafting of the resolutions chose to change that to "clearing" in an attempt to make it more easily understood by the general public, but it appears that the attempt was not successful and has complicated things instead of simplifying them. Some of the delegates appeared to be confused, though it is sometimes hard to tell whether a particular delegate was really confused or was being some sort of a devil's advocate.

    It's the nature of the beast that this criterion requires explanation in detail that is mathematically beyond the working knowledge that most of us have in orbital mechanics. My OP in this thread is aimed at the general public, and I stand by my opinion that presenting it that way is better than simplistically saying "has cleared its orbital neighborhood". It takes more time and effort but gives the public an honest statement. Those who don't understand the dynamic basis of it but wish to learn more can enroll in a suitable astronomy major curriculum. Trying to learn it by browsing online can be dicey because of the risk of encountering garbage.
    Please don't take this as belittling those of us who are not inclined toward such studies. With the critical thinking skills we can get from a good general education that includes appropriate studies in science and mathematics, we can listen to experts and be confident that they know what they are talking about. If our education is poor, we will have much worse problems than merely having trouble with concepts of orbital dominance dynamics.

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    Quote Originally Posted by George View Post
    So would a few percent variance between 1 billion and 4 billion years be arbitrary?
    Eh? If the range or margin is 400%, I'd say arbitrariness is a lesser concern to usability.

    In a thousand years of technological advancements, this should be considered. Hopefully, before then, we can ask a more advanced civilization how they define planets.
    For objects orbiting distant stars, it may have to wait that long, but for objects in our own solar system, we can start today. I just have.

    How big a kick? A Pogo stick should be used, with an accurate one kept in Paris.
    Good question. It might be limited to unenhanced human motion. Make it a high jump. Say its on the equator so that rotation adds to it if you want, to increase the size and mass of the reference sphere. Say the surface has to be solid enough that a kick doesn't just slush the gravel pile. If we have to make arbitrary decisions, they may as well be human scale and relatable. So, if an object is massive enough that people can live and work on the surface using non-rotating habs (a locomotion concern, not a health concern), as opposed to hollowing it out and living on the inside centripetally, then we can go with that. It may be arbitrary, but it makes sense for our species-centric needs.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by Hornblower View Post
    As can be heard in the Session II video, the third one in the presentation linked above, the original language of the proposed resolution was about gravitational dynamic dominance. The group responsible for the drafting of the resolutions chose to change that to "clearing" in an attempt to make it more easily understood by the general public, but it appears that the attempt was not successful and has complicated things instead of simplifying them. Some of the delegates appeared to be confused, though it is sometimes hard to tell whether a particular delegate was really confused or was being some sort of a devil's advocate.

    It's the nature of the beast that this criterion requires explanation in detail that is mathematically beyond the working knowledge that most of us have in orbital mechanics. My OP in this thread is aimed at the general public, and I stand by my opinion that presenting it that way is better than simplistically saying "has cleared its orbital neighborhood". It takes more time and effort but gives the public an honest statement. Those who don't understand the dynamic basis of it but wish to learn more can enroll in a suitable astronomy major curriculum. Trying to learn it by browsing online can be dicey because of the risk of encountering garbage.
    "Dominance" is still an arbitrary criterion, and it still leaves the distinction between "planet" and "dwarf planet" as location dependent, which, in my opinion, makes it an essentially invalid criterion.

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  15. #135
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    Quote Originally Posted by swampyankee View Post
    "Dominance" is still an arbitrary criterion, and it still leaves the distinction between "planet" and "dwarf planet" as location dependent, which, in my opinion, makes it an essentially invalid criterion.
    My bold. I would say "undesirable" rather than "invalid" should that be the case. While the location dependence can create perceptual problems for the public at large, it does not necessarily invalidate the scheme in terms of scientific usefulness. What I find to be frivolously arbitrary is insisting on saying "a dwarf planet is not a planet." Having seen the video, it now appears that the framers of the resolution really were insisting on that language. I stand by my opinion that "major planet" (my preference) or "classical planet" (the defeated amendment) would be a perfectly good expression for the big eight, with "dwarf planet" or "minor planet" for large main belt asteroids and KBOs that are not massive enough to dominate their respective neighborhoods. Having them semantically under the umbrella of "planets" does not impair the scientific utility of the classification scheme, and anyone with even a rudimentary education should be able to adapt to revisions in the terminology as needed with continuing discoveries. That goes for explaining the reclassification of Pluto to the public. If asked about how many planets are in our solar system, I could say, "Eight major planets, along with (however many to the best of our current knowledge) dwarf planets. If there are 40 or more of the latter, so be it. We could say that Pluto is the first trans-Neptunian dwarf to be discovered.

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    Quote Originally Posted by Hornblower View Post
    My bold. I would say "undesirable" rather than "invalid" should that be the case. While the location dependence can create perceptual problems for the public at large, it does not necessarily invalidate the scheme in terms of scientific usefulness. What I find to be frivolously arbitrary is insisting on saying "a dwarf planet is not a planet." Having seen the video, it now appears that the framers of the resolution really were insisting on that language. I stand by my opinion that "major planet" (my preference) or "classical planet" (the defeated amendment) would be a perfectly good expression for the big eight, with "dwarf planet" or "minor planet" for large main belt asteroids and KBOs that are not massive enough to dominate their respective neighborhoods. Having them semantically under the umbrella of "planets" does not impair the scientific utility of the classification scheme, and anyone with even a rudimentary education should be able to adapt to revisions in the terminology as needed with continuing discoveries. That goes for explaining the reclassification of Pluto to the public. If asked about how many planets are in our solar system, I could say, "Eight major planets, along with (however many to the best of our current knowledge) dwarf planets. If there are 40 or more of the latter, so be it. We could say that Pluto is the first trans-Neptunian dwarf to be discovered.
    My reading of the IAU definition is that a dwarf planet is not a member of the larger set of planets, but a distinct category. The IAU's abuse of English is a different issue than that of their location-dependent classification scheme.

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    Quote Originally Posted by swampyankee View Post
    "Dominance" is still an arbitrary criterion, and it still leaves the distinction between "planet" and "dwarf planet" as location dependent, which, in my opinion, makes it an essentially invalid criterion.
    Any classification scheme is arbitrary. However some are useful.

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    Quote Originally Posted by glappkaeft View Post
    Any classification scheme is arbitrary. However some are useful.
    Exactly. If, however, a taxonomy, or mere labeling, is effective and not capricious, then that may be all that is needed. Consider stellar classifications, including Flemming's & Maury's (revised to MKK), along with Cannon's, these all are arbitrary. Even the oddity (non-alphabetic) of the OBAFGKM system seemed helpful to many astronomers (including those on the committee back then) regarding implementing this Draper classification globally because it did not argue for any particular theory. [The favored view at that time was the red to blue color change for stars was an evolutionary process with some saying the red phase was the youngest and others suggesting the blue phase as the earliest. I think Russell was one of the earliest to look at mass instead.]
    Last edited by George; 2017-Mar-10 at 10:09 PM. Reason: grammar
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    One thing -- perhaps the most important thing -- for a taxonomy scheme is consistency. I think the IAU's scheme fails that in regard to objects: Ceres and Pluto are both "dwarf planets," but Ceres and Pluto have much less in common than do Mercury -- a "planet" -- and the dwarf planet Ceres. In its way, I think, it's more similar to the popular classification of whales and dolphins as fish -- they all swim -- or considering all small, terrestrial arthropods as "bugs," regardless of number of legs.

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    Here's a twist. Some people want to restore Pluto's planethood, like Alan Stern and some other members of the New Horizons team. That redefinition is in A Geophysical Planet Definition, and it's essentially the second part of the IAU's recent definition. That is having an approximate hydrostatic-equilibrium shape, meaning being approximately spherical or approximately a flattened sphere.

    This redefinition would also not only make Pluto a planet, but also some 110 other Solar-System objects, including the Earth's Moon and several other moons. Every round object in the solar system under 10,000 kilometers in diameter, to scale | The Planetary Society

    Their objections to the IAU definition:
    First, it recognizes as planets only those objects orbiting our Sun, not those orbiting other stars or orbiting freely in the galaxy as “rogue planets.” Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs near Earth. Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth sized object in the Kuiper Belt would not clear its zone.
    The first one does not seem like a big issue to me, since it is easy to extend the IAU's definition to outside the Solar System. It seems to me a misstatement more than anything else.

    The zone-clearing part is a more difficult issue, it must be conceded.

    More: Pluto became a dwarf planet 9 years ago today -- but that's 'bulls---,' according to a leading NASA scientist | Business Insider

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    Quote Originally Posted by swampyankee View Post
    One thing -- perhaps the most important thing -- for a taxonomy scheme is consistency. I think the IAU's scheme fails that in regard to objects: Ceres and Pluto are both "dwarf planets," but Ceres and Pluto have much less in common than do Mercury -- a "planet" -- and the dwarf planet Ceres. In its way, I think, it's more similar to the popular classification of whales and dolphins as fish -- they all swim -- or considering all small, terrestrial arthropods as "bugs," regardless of number of legs.
    Exactly. For kids who weren't even alive when Pluto was a planet, the number 1 fact they know about Pluto is that it's not a planet. I tell them it's dwarf planet and have to take quite a bit of time to explain it. I tell them Ceres is a dwarf planet, too. By definition, that means they are both round and orbit the Sun in a region with a lot of other objects similar to them. But it tells me nothing about their composition or where they are or how they formed--the really important things about them. So I have to say Ceres is also an asteroid, and Pluto is also a kuiper belt object. And then explain what that means. In my opinion, Pluto should be a large round Kuperiod, and Ceres should be a large round asteroid. They shouldn't also be dwarf planets.

    But think of this: to the completely ignorant, how is this case any different than saying Neptune and Mercury are both planets? There is still so much different about them that we have to go into a more detailed level of description. Whether a thing is a "planet" or not, I really don't care. We just need to group similar things into categories. Let's put more focus on rocky, gaseous, ice...let the prime designation be more descriptive.

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    Quote Originally Posted by lpetrich View Post
    Here's a twist. Some people want to restore Pluto's planethood, like Alan Stern and some other members of the New Horizons team. That redefinition is in A Geophysical Planet Definition.
    Here's my 2 cents worth on this idea...

    In the decade following the supposed ‘demotion’ of Pluto by the International Astronomical Union (IAU) [1], many members of the public, in our experience, assume that alleged “non-planets” cease to be interesting enough to warrant scientific exploration, though the IAU did not intend this consequence.
    "Non-planets" may be less interesting but public interest is stirred by the interest an object has by scientists, media, or themselves. The moons of Jupiter have certainly gained a lot of public interest and they are rarely called planets. [Their use of “small planets” (mentioned in their “Common Usage” paragraph) to argue for the use of “planets” seems actually counter to their argument since “small” is also a way to inform the reader the object is not a normal “planet”, especially since much is discussed about the use of "small" (dwarf) makes a big distinction.]

    Our definition captures the common usage already present in the planetary science community.
    Even if this is true, in the public perception, “planet” is not a word that describes over 100 objects in the solar system, and I doubt the public would wish this if asked explicitly.

    Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs near Earth
    There’s a reason they are called “planet-crossing orbits”, right? “Planet” gradually became commonly used as a sized-object when Herschel proposed it in 1802. The discovery of the little “planets” Ceres and Pallas suggested it was time for a new word to de-rate them. Piazzi, who discovered Ceres, wrote Herschel to ask, “And for the naming, could one not call the little planets ‘planetoids’?” [from here

    The IAU definition calls for a planet to be massive enough to clear, or dominate, the orbit. Trojans, Greeks, and planet crossing objects would not change the mass needed to toss smaller objects out of the orbit. This mass is quantifiable.

    Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone.”
    True, but there is no rule saying you can’t use extrinsic properties to define something, especially if it is reasonable to do so. How many biological species names are completely intrinsic? “Pygoscelis antarcticus” is a fairly geographically confining name for this penguin.

    This is where the alternative choices for “planets” must be considered since it would be nice to keep it as intrinsic as possible. The alternatives don’t seem to offer a better fit, however, including theirs.

    For example, even an Earth sized object in the Kuiper Belt would not clear its zone.
    False. The Earth would clear its orbit as far out as about 400 AU, or 8x farther than the outer Kuiper Belt. Not in one orbit, of course, so time is a factor, admittedly, but not a big issue.

    We eschew this inconsistency. Thus, dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth’s Moon are ‘full-fledged’ planets.
    Really? Only if the Moon is made of cheese will calling it a planet be palatable. As noted earlier, astronomers took these objects, originally called planets, and correctly changed their classification name, even if not officially. Why would we want to reverse their work? There is more evidence today that their naming efforts make even more sense today, and for essentially the same reason – too many little guys.

    Astronomers with research interests in dynamics may find the IAU definition perfectly useful. However, many planetary scientists are closely aligned with the geosciences. Accordingly, our geophysical definition is more useful for planetary geoscience practitioners, educators, and students.”
    Perhaps this is a good argument for “dwarf planets” to be a sub class of planets.

    With the above definition of a planet, we count at least 110 known planets in our Solar System (Figure 1). This number continues to grow as astronomers discover more planets in the Kuiper Belt [e.g., 7]. Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn only a few (9? 12? 25?) planets of interest. For an analogy, there are 88 official constellations and ~94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets.
    But we already have a name for the planets of interest – “planets”. Constellations, elements, and the number of piano keys for that matter, have a different story and, more importantly, a different history. If we want to wipe the slate clean, we should eliminate “planet” altogether since these objects aren’t stars that are wandering.
    Last edited by George; 2017-Mar-13 at 07:11 PM.
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    When did Earth become a "planet"?
    Et tu BAUT? Quantum mutatus ab illo.

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    I'll make a list of what was considered planets:
    • Premodern:
    • -- The Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn
    • 1650 - 1700: heliocentrism widely accepted: the Sun and the Moon were demoted and the Earth was promoted.
    • -- Mer, Ven, the Earth, Mar, Jup, Sat
    • 1781: Uranus discovered.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Uranus
    • 1801: Ceres, 1802: Pallas, 1804: Juno, 1807: Vesta discovered.
    • -- Mer, Ven, Ear, Mar, Ceres, Pallas, Juno, Vesta, Jup, Sat, Ura
    • 1846: Neptune discovered.
    • -- Mer, Ven, Ear, Mar, Cer, Pal, Jun, Ves, Jup, Sat, Ura, Neptune
    • Starting in 1845, many more asteroids discovered. They get demoted from planethood, including the first four.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep
    • 1930: Pluto discovered.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep, Pluto
    • 1992: first additional Trans-Neptunian Object discovered; several other ones discovered soon after.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep, Plu, some other TNO's?
    • 2006: the IAU demotes Pluto and other TNO's.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep

    So there you have it. Planets have come and gone over the years and decades and centuries.

    This new definition would not only restore planethood to Pluto, it would also do so for the Moon, though it would not do so for the Sun.

    I think that we need some generic term for a celestial body in approximate hydrostatic equilibrium. I've seen "orb".

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    Quote Originally Posted by lpetrich View Post
    I'll make a list of what was considered planets:
    • Premodern:
    • -- The Sun, the Moon, Mercury, Venus, Mars, Jupiter, Saturn
    • 1650 - 1700: heliocentrism widely accepted: the Sun and the Moon were demoted and the Earth was promoted.
    • -- Mer, Ven, the Earth, Mar, Jup, Sat
    • 1781: Uranus discovered.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Uranus
    • 1801: Ceres, 1802: Pallas, 1804: Juno, 1807: Vesta discovered.
    • -- Mer, Ven, Ear, Mar, Ceres, Pallas, Juno, Vesta, Jup, Sat, Ura
    • 1846: Neptune discovered.
    • -- Mer, Ven, Ear, Mar, Cer, Pal, Jun, Ves, Jup, Sat, Ura, Neptune
    • Starting in 1845, many more asteroids discovered. They get demoted from planethood, including the first four.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep
    • 1930: Pluto discovered.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep, Pluto
    • 1992: first additional Trans-Neptunian Object discovered; several other ones discovered soon after.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep, Plu, some other TNO's?
    • 2006: the IAU demotes Pluto and other TNO's.
    • -- Mer, Ven, Ear, Mar, Jup, Sat, Ura, Nep

    So there you have it. Planets have come and gone over the years and decades and centuries.

    This new definition would not only restore planethood to Pluto, it would also do so for the Moon, though it would not do so for the Sun.

    I think that we need some generic term for a celestial body in approximate hydrostatic equilibrium. I've seen "orb".
    My bold. If we use "promoted" and "demoted" in describing changes in the classification scheme, I would say Earth was demoted from its original unique central body status to being just another planet, and the Sun was promoted from being just another planet to being a supreme central body.

  26. #146
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    I made this figure for a class on exoplanets last year:

    planet_counting_label.png

  27. #147
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    What Hornblower bolded: 1650 - 1700: heliocentrism widely accepted: the Sun and the Moon were demoted and the Earth was promoted.
    Quote Originally Posted by Hornblower View Post
    My bold. If we use "promoted" and "demoted" in describing changes in the classification scheme, I would say Earth was demoted from its original unique central body status to being just another planet, and the Sun was promoted from being just another planet to being a supreme central body.
    I'll compare me and you on planethood changes
    • Sun: demoted from planethood / promoted from planethood to become the central body in the Solar System
    • Earth: promoted to planethood / demoted from Solar-System centrality to being just another planet
    • Moon: demoted from planethood / demoted from planethood

  28. #148
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    Quote Originally Posted by George View Post
    Here's my 2 cents worth on this idea...

    True, but there is no rule saying you can’t use extrinsic properties to define something, especially if it is reasonable to do so. How many biological species names are completely intrinsic? “Pygoscelis antarcticus” is a fairly geographically confining name for this penguin.
    It's the penguin's name, not the penguin's classification. Organisms can be named after locations where the type specimen was found, colors, legendary beings, physical characteristics, funding agencies, or celebrities. True, there are no natural penguin populations north of the equator, but that's not because of how they classify birds, it's because the birds that fit the penguin taxonomy don't live north of the equator, not that the system of taxonomy is defined so penguins only live south of the equator and pseudo-penguins north of it.

    Quote Originally Posted by George View Post
    Here's my 2 cents worth on this idea...

    False. The Earth would clear its orbit as far out as about 400 AU, or 8x farther than the outer Kuiper Belt. Not in one orbit, of course, so time is a factor, admittedly, but not a big issue.
    But, according to the graphic provided by Margot (http://mel.ess.ucla.edu/jlm/epo/planet/planet.html), neither Mars nor Mercury would be planets much beyond 40 AU. Saying the Earth would still be a planet out there is fine and dandy, but doesn't address the main issue: that an identical object would be classified differently depending on location. Similarly, Pluto would be a planet if its semi-major axis was less than about 1 AU. I think one can accept the IAU both as a fait accompli and as intrinsically flawed. One example of a flaw is that Ceres and Pluto are radically different objects, but have the same classification. Stern's geophysical method is much more sensible. Something based on composition would be much more sensible. The IAU definition is a fait accompli. It still stinks.

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  29. #149
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    Quote Originally Posted by lpetrich View Post

    I think that we need some generic term for a celestial body in approximate hydrostatic equilibrium. I've seen "orb".
    I've seen Emily Lakdawalla and others use the term "worlds" for the large and large-ish round bodies, which include almost all the moons. When you get to things like Hyperion, Phobos, Deimos, and even Atlas and Pan, I think you still run into issues, but overall I like the term.

    Like many others, I also dislike "dwarf planet" as a category for objects as different in their apparent history as Pluto, Eris, Orcus etc. and the large asteroids like Ceres and Vesta. It is hard to argue against the fact that the terrestrial planets are different than the gas and ice giants (which may be vastly different than each other, too), though. So you're left with either an arbitrary and messy "size and influence" scheme or an astro/geophysical scheme. I try to take the view that it doesn't (or shouldn't) matter as much what we call them, so long as we study and learn from them.

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    Isn't just a hunch or guess
    It's more like a question
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    -They Might Be Giants, "Science Is Real"


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  30. #150
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    Quote Originally Posted by Hornblower View Post
    My bold. If we use "promoted" and "demoted" in describing changes in the classification scheme, I would say Earth was demoted from its original unique central body status to being just another planet, and the Sun was promoted from being just another planet to being a supreme central body.
    While Earth was the center of the known universe, it wasn't some sort of apex. It was more the basement, the one step above Hell. An analogy is the drain in your shower: all the water swirls around it, but it's just a stop on the way to the sewers.

    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



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